In-line tunnel-type cryogenic food freezers, such as those sold under the trademark CRYO-QUICK by Air Products and Chemicals, Inc., utilize liquid nitrogen (LIN) as an expendable refrigerant. Examples of such freezers are shown and described in U.S. Pat. Nos. 3,403,527, 3,813,895, 3,892,104, 4,229,947, 4,475,351 and 4,800,728. Freezers of the type shown in the aforementioned patents can achieve high thermal efficiency because they are designed as counterflow heat exchangers. Liquid cryogen (e.g., liquid nitrogen or LIN) is sprayed onto the product being refrigerated adjacent to the discharge end of the freezer or tunnel. The cold nitrogen gas at -320.degree. F. (-196.degree. C.) evolved in the liquid nitrogen spray zone, moves through multiple zones of gas recirculation as it flows to the entrance end of the freezer. Since the maximum available refrigeration has been utlized at that point, the warm nitrogen gas can be vented to the outside atmosphere by an exhaust blower.
Current tunnel-type food freezers sold under the CRYO-QUICK trademark include a liquid nitrogen control system such as described and claimed in U.S. Pat. No. 3,613,386. The amount of liquid nitrogen introduced into the freezer utilizing the '386 control system is controlled by the liquid nitrogen spray header pressure and the recirculating gas temperature. As a result, the control system will maintain the total heat transfer capability of the freezer at a constant rate during production.
The present control system has a disadvantage during the initial cooldown of the freezer. Since the recirculating gases at ambient temperature, e.g., 70.degree. F. (21.degree. C.), the cryogen flow control valve will open fully. Under these circumstances, the liquid nitrogen may not vaporize completely and may leak from the freezer.